Originally Posted by Jeroen1000
1. How exactly do you calculate pattern size?
Window size is usually discussed as percent area. Take the height of the window times the width of the window, and divide by the height of the screen times the width of the screen. That will give you a decimal, which could be multiplied times 100 for a percentage.
(H1*W1)/(H2*W2)*100 = percent area
2. What is the difference between the APL (average picture level) patterns and the "regular" ones?
The APL patterns from AVS HD 709 are similar to bar patterns, but they're easier to measure. All the video levels for measurements are on-screen at the same time, so generally average brightness of the image is expected to remain constant for all the measurements. With the APL patterns the intent is to have a constant average brightness during measurements.
A typical window usually has only two video levels, the level to be measured and the surrounding black. Usually the center area of the window remains the same size, so as the video levels increase in brightness from black to white the average brightness of the image also increases. With typical windows or fields the average brightness of the image varies depending on the video level being measured.
3. There has been talk that a problem is that patterns are always displayed within a black rectangle. Is this so?
Not really, the main difference between the patterns has to do with average brightness. The APL patterns have a constant average brightness, and with standard windows or fields average brightness varies. Various displays react differently as average brightness changes (see Note). Some displays will deliver a similar light output regardless of average brightness, and other displays will vary light output as average brightness changes. Here are two video patterns to quickly look at how average brightness may affect the image:
1) AVS HD 709 Misc. Patterns A5, Dynamic Brightness
2) Avia II "Black Level Bars + Steps + Varying Gray" pattern
4. Can someone describe how we can detect when the ABL (automatic brightness limiter) kicks in?
Probably the easiest thing to do would be to measure the display response, but I get the impression not many people have done that. There are a couple ways you could look at how the display deals with a constant video level and a changing average brightness. One way would be to use a constant measurement area and then increase the brightness of the background (the Avia II pattern mentioned above follows this concept). Another way would be to measure what happens as the area of the measurement pattern increases. For example ColorHCFR will let you display windows of various sizes, and you could measure what happens to Y as you increase the window from a very small area to a larger area.
5. Could someone link the term "stimulus" to this? I understand it ranges from 0 to 100. IRE is a synonym for it? Is there some correlation between stimulus and pattern size?
The percent markings on the patterns just indicate video levels. The video level is independent from pattern size. You can have a white video level that is used for a very small window, and you can have a white video level that is used in a very large window. The small window size has a lower average brightness than the larger window, although the two windows use the exact same video level.
The patterns above marked 1 and 2 have some bars on the screen that always use the same video levels. Part of the screen changes shade in those patterns, which causes average brightness to change. The bars from the mentioned patterns may change light output (change brightness) as a result of the average brightness changing, even though the video levels have remained the same. This sort of change in light output independent from video level is the main disconnect between gamma theory described by Charles Poynton and typical application.
Note: In fact some displays work in rather opposite directions when it comes to how they deal with average brightness. Lets say you have a Samsung LCD with dynamic backlighting and you also have a Samsung plasma. Both TVs are showing the same picture with some white in the image. Lets say the image is from a camera on a train, the white is from a light on a train, and the train drives into a tunnel. So you have some white in the image and the scene transitions between a somewhat bright scene and a dark scene. The white on the LCD may get darker on the transition to the dim scene, while the white on the plasma might get brighter as the scene gets darker. In both cases you have the exact same input (video levels), but how the displays react is not necessarily similar.